A radiation therapy device controller identifies a position in a living body to be irradiated with radiation based on a position of a diseased portion (tumor) displayed on a CT image (a computed tomography image) and irradiates the position with radiation.
The diseased portion, such as a tumor in the living body, varies under the influence of respiration or the like. Therefore, it is necessary to identify a diseased portion whose position varies in each of CT images that are generated sequentially over time and identify the position to be irradiated with radiation based on the position of the diseased portion.
Here, in order to improve identification accuracy of the position to be irradiated with radiation, it is necessary to accurately identify the diseased portion within the living body using the CT image. If the image quality of the CT image is improved, it is possible to accurately identify the position of the diseased portion and accurately identify the position to be irradiated with radiation by tracking the identified diseased portion. Methods of generating a CT image include an FBP method (Filtered Back Projection method).
The FBP method is an algorithm used to generate a general CT image, and is technology of reversely projecting X-ray projection data through a fast Fourier transform to perform image reconstruction. However, although there is an advantage in that a computation time for generation of the CT image using the FBP method is short, deterioration of image quality such as generation of artifacts in the image becomes a problem particularly if the amount of projection data used for reconstruction is small. Here, CT image generation technology for resolving a problem of degradation of image quality includes a successive approximation method (an Iterative Reconstruction method). Patent Documents 1 and 2 disclose the technology of the Iterative Reconstruction method.